This invention relates to a drilling tool with rollers, such as a bicone, a tricone, etc. with cutting characteristics that are highly improved compared to tools of the prior art.
To simplify matters reference will only be made to tricones, i.e. drilling tools with three rollers, but the invention also applies to drilling tools with any number of rollers.
A known tricone 10 is shown in FIGS. 1 and 2, and includes a tubular steel body 12 mounted to the end of a tubing, not shown, which drives it in rotation around its axis x--x. In the lower opening of the body three conical steel rollers 14, 16, 18 are mounted to freely rotate, their tops being oriented towards the axis of the tool. The rollers have bearing rings 20 fitted with rows of teeth 22 that can either be tooled directly in the rings and then covered with a coat of tungsten carbide, or comprised of tungsten carbide cutting points mounted inside holes that are tooled in the walls of the rings.
As shown in FIG. 2, the axes of the rollers do not exactly converge on the axis x--x of the tool, but instead they are spaced respectively by a distance d, hereafter the "inter-axis". This shifting can also be characterized by the angular shift angle .alpha. that the axis of the roller creates with the radial plane of the tool that contains the center of the base of the roller. Due to this shift, the rollers exert a scraping or abrading effect on the formation rock, and the greater the angle .alpha., the more energetic this effect is.
When the rollers 14, 16, 18 are in contact with the rock, the rotation of the tool in the direction of arrow f in FIG. 2 drives the three rollers in the direction of the circular arrows indicated in the Figure.
There are two kinds of tools with rollers:
1) Tools whose rollers have a high angular shift. These tools are efficient for drilling soft or medium-soft rocks, but less so when hard rocks need to be drilled, because as a result of scraping the rock, the teeth or cutting points of the rollers wear out quickly. The tools according to this first kind destroy the rock with the three following actions:
through impact, each time a new tooth 22 strikes the rock,
through puncturing, under the effect of the axial force F.sub.v which is exerted on the tool 10, and
through abrading-shearing as a result of the scraping of the cutting points on the rock.
2) Tools whose rollers have a small or no angular shift, in which case the rock is destroyed only by the first two effects mentioned above. The cutting points in this second category can be advantageously diamond-charged, but the efficiency of the tool is greatly reduced by the fact that there is no destruction through scraping.